The present invention relates to a method and device for processing video frames for stereoscopic display on a display device, particularly a Plasma Display Panel (PDP), having a plurality of luminous elements. In each case one or more of them belongs to a pixel of a video frame. Wherein each video frame includes a left and a right picture and the time duration of the video frame is divided into a plurality of sub-fields during which the luminous elements can be activated for light emission/generation in small pulses corresponding to a sub-field code word which is used for brightness control.
The 3D perception from the Human Visual System (HVS) is based on the close side-by-side positioning of the eyes. Each eye takes a view of the same area from a slightly different angle. These two separate images are sent to the brain for processing according to FIG. 1. When the two images arrive simultaneously in the back of the brain, they are united into one picture. The mind combines the two images by matching up the similarities and adding the small differences to catch finally a three-dimensional stereo picture. With stereo vision, the HVS sees an object as solid in three spatial dimensions (width, height and depth) and it is the added perception of the depth dimension that makes stereo vision so rich and special. Moreover, a stereo picture will increase the impression of sharpness in the brain.
In video technology 3D images are generated with the help of two video cameras positioned side-by-side similar to the human eyes. Other methods mainly based on complex software are also able to generate artificial stereo pictures by ray tracing (simulation of light propagation). These images shall be called, left and right images. If right and left images are displayed sequentially from a source according to FIG. 2, and a synchronized shutter system in front of the eye allows the right image to only enter the right eye and conversely for the left eye, then the stereovision can be observed. The shutter can be mounted in glasses that are matched with a display in which two constituent pictures are presented in alternation instead of simultaneously. The glasses occlude one eye and then the other in synchronism with the image displaying. This method is often called “field sequential”. This method avoids the retinal rivalry caused by anaglyph viewing (other method based on a two-color glasses associated with a two-color picture—each color related to one eye and resulting in a monochrome stereoscopic vision, very old method traced back to 1858). Nevertheless, this method can introduce other discomfort such as the introduction of time parallax between the two images, or the possibility of “ghosting” between the image due to phosphor persistence.
Most glasses-shutter systems use LCDs that work with polarized light. Currently, glasses using LCDs can provide good switching speed and reasonable extinction of the alternative lenses. The electro-optical polarizing shutters available on the market today transmit only 30% of the unpolarized input light (rather than 50% for perfect polarizers) and this reduces a lot the image brightness. Some eyeglass shutters are connected by wires to the monitor, others are controlled by infrared and are wireless.
The display of stereo pictures on a Plasma screen is not a simple matter of design choice, because it needs to display two different pictures per frame period which is a new challenge for this technology if one does not want to accept a great reduction of frame repetition frequency.
A PDP utilizes a matrix array of discharge cells which can only be “ON” or “OFF”. Also unlike a CRT or LCD in which gray levels are expressed by analog control of the light emission, a PDP controls the gray level by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye time response. To perform a grayscale rendition, the Plasma display is commonly divided in sub-lighting periods called sub-fields each one corresponding to a bit of the input video picture data. For instance, if 8 bit luminance levels are provided, in that case each level will be represented by a combination of the 8 following bits:1-2-4-8-1-3-6-128.
To realize such a coding with the PDP technology, the frame period will be divided in 8 lighting periods (called sub-fields), each one corresponding to a bit. The number of light pulses for the bit “2” is the double as for the bit “1”, and so forth. With these 8 sub-periods, we are able through sub-field combination, to build the 256 gray levels.
For clarification, a definition of the term sub-field is given here: A sub-field is a period of time in which successively the following is being done with a cell:                1. There is a writing/addressing period in which the cell is either brought to an excited state with a high voltage or left in its neutral state with lower voltage.        2. There is a sustain period in which a gas discharge is made with short voltage pulses which lead to corresponding short lighting pulses. Of course only the cells previously excited will produce lighting pulses. There will not be a gas discharge in the cells in neutral state.        3. There is an erasing period in which the charge of the cells is quenched.        
In some specific plasma driving schemes (incremented coding, proposed by Pioneer) the addressing or erasing periods are not present in each sub-field. Instead, a selective addressing/erasing is performed ahead or after a group of sub-fields.
A simple method to implement a stereoscopic display is based on the use of LCD shutter glasses and the separation of sub-fields into Left(L) and Right(R) sub-field groups which are synchronized with the opening and closing of the LCD shutter glasses. It is a further advantage of this method that with the same display 2D and 3D pictures can easily be generated by a change of the sub-field encoding process.
For the following explanations, we will make the assumption that the PDP is able to display 20 sub-fields per frame in 60 Hz mode (16.67 ms frame period). In addition we will also make the assumption that the temporal response of the shutter eyeglasses need the time of one addressing period. Obviously, all these values are only an example!
FIG. 3 shows a light emission scheme according to the assumptions made above. Ten sub-fields are assigned to each of the left and right images, for example. The numbers on top of the sub-fields denote the relative sub-field weights. The total sum of the sub-field weights is equal to 255 corresponding to the highest possible 8-bit value. In video technology the input RGB data words are 8 bit numbers that is sufficient for Standard TV quality (SDTV). The addressing periods of the sub-fields are shown in FIG. 3 but the erasing periods are not shown as they are much smaller than the addressing periods. With a 10 sub-field code, the quality of both right and left images will be good.